We turned to Likester, a social-media analytics company that tracks Facebook engagement. It has tracked more than 10 million Facebook photos, tallying the number of likes and comments for all of them.

They shared the 100 most viral photos with us. Most were from 2012, which makes sense since Facebook now has more than 1 billion users.

We screened the collection for photos that were, in our opinion, spam—for example, any photo whose caption explicitly asked someone to like or comment on the image. By eliminating those, we winnowed it down to pictures that were truly liked by millions of people.

All of them evoke some sort of feeling. Many were accompanied by emotional stories.

It's safe to say Barack Obama provokes the most reaction on Facebook. Three of his photos made the list.